Lux for example has 11 Skins, of course she is way older than f.e. Ornn, but she keeps getting skins while he's there with a single skin. Ornn meanwhile out for 2 years, didn't get a single skin since release. Similar things with many other champs.
And don't come to me with "That champ is more frequently played" ... Of course it is, if it gets full support and attention of the marketing team :/